Accompanying the development of optical technology in recent years, light has come to be used in such fields as optical communications, optical sensing, and optical computers, and there is no doubt that these fields of use will continue to expand in the future. Under these conditions, the measurement of light has become extremely important. Tools used for measuring light, such as photomultiplier tubes and the like, are well known in the art. By using the photomultiplying effect of photomultiplier tubes, it is possible to detect weak light with great efficiency.
However, in order to expand the new fields for using optics in the future, it will be extremely important to measure light at the photon level in time and space, as in “when and how many photons have arrived.” The conventional tool (method), such as a photomultiplier tube, for measuring light is not a tool for measuring light in time and space at the photon level.